People go out a bunch of times during a month and have reason to check in on Foursquare

There are many great apps people use that don't have this first many like TaskRabbit, Groupon and Uber. Those are services you may use every once in a while but most will users won't use them frequently enough to satisfy the first many.

However, you can do things to encourage your users to take more actions on your app. For example:

Instagram gave people tools to take nicer pictures causing them to take more pictures

Foursquare has done lots of work to make checking-in as fast as possible to encourage more check-ins

Socialcam realizes that people may not have many videos to take and share a month and have thus been seeding their content with YouTube videos like the Dark Knight trailer

The Second Many

The second many referes to a user wanting to share the action they take on your service with not just one friend but many friends (i.e, push to Facebook).

For example:

People broadcast their Instagram pictures to their friends on Facebook

People push their Pinterest pins to all their friends on Facebook

People share their Foursquare check-ins on Facebook

As with the first many, there are lots of successful apps where users don't share their actions with many friends. For example, most people don't share their TaskRabbit tasks, Groupon deals or Uber rides with all their friends but may coordinate with a specific friend or spouse.

There are ways to get users to push their actions more consistently to Facebook. Services like Socialcam and Spotify have been very aggressive on this front. They don't ask users to specify, for each action, whether it will get pushed to Facebook (like Instagram and Foursquare do). Instead, they default to having all actions posted to Facebook. While questionably aggressive, it clearly helps them satisfy the second "many".

Many-to-Many Sharing

When both sides of the expression get into the "many" territory, assuming it's not a niche product, you may experience "viral" growth.

This growth is very powerful and it may be worth re-working how your product works to encourage the "many" on both sides of the equation. Clearly, Socialcam and Spotify have done so.

As a word of caution, while it will may help you get to millions of users, this doesn't speak to retention. It's likely that the users signing-up because they saw something on a friend's Facebook feed are much less likely to turn into real long-term users. Also, the more aggressive you are with pushing their actions to Facebook, the more angry you may make your users.